Selective radiators have received considerable research attention and been extensively applied in many areas, such as thermophotovoltaic systems, infrared cloaking and sensing. Herein, the design of the infrared selective radiator based on ultrathin metallic films was fully demonstrated. From the discussions on the radiative characteristic and growth law of the metallic film, it has been concluded that a continuous ultrathin metallic film can behave as a thermal radiation enhancer. Taking the radiative cooling technology as the application background, a multilayered broadband selective radiator, consisting of an ultrathin Ag film and dielectric Ge films is designed and fabricated. The proposed radiator, which can radiate selectively in the 8-13 gm atmospheric transparency window (epsilon(5-8) mu m = 0.21; epsilon(8-13) mu m = 0.84; epsilon(13-25) mu m = 0.39), achieves an average temperature reduction of 3.5 degrees C under outdoor conditions and exhibits potential application in radiative cooling technology. Moreover, for our proposed selective radiator, flexible tuning of radiative characteristics can be achieved via structural design, which paves the way for its application in various areas. Unlike the common metamaterial based thermal radiator, our selective radiator with a planar structure is free from the complicated process of lithography or etching and is scalable to realize large-area fabrication and application.